The objective of this study is to determine duration and characteristics of naturally acquired immunity to corona-, respiratory syncytial, and parainfluenza viruses, and Mycoplasma pneumoniae and to evaluate neonatal Chlamydia trachomatis infection as a precursor of chronic sequelae such as excess of pneumonia and bronchitis. Data on illnesses including respiratory specimens and sera have already been collected, as well as data concerning illnesses from more than 200 families under intensive surveillance for respiratory infections and related illnesses from 1965-1969 and another 210 families under observation from 1975-1979. Of the first group of families, 65 re-entered observation by annual bleedings 1972-1975 and 32 took part in the 1975-1979 study. Specific questions to answer include 1) frequency of reinfection with the above agents in persons long followed serologically, 2) the relationship of specific antibody to reinfection after known within-family exposure, 3) the symptomatology of reinfection, 4) long-term effects of neonatal infection with C. trachomatis and RS virus on health status, 5) epidemiology of coronavirus infection, and 6) establish if M. pneumoniae infection early in life predisposes to more severe manifestations at later exposure. Also, the infection experience and immunity status of two groups of children, those frequently reported ill and those rarely ill, will be compared. Serologic analysis will be primarily with ELISA and complement fixation tests. Virus isolation will be attempted in VERO and EE cell lines, as well as by ELISA technique for (antigen) virus capture. (Specimens have previously been analyzed for influenza and rhinoviruses.) The long term objective is to provide epidemiologic and immunologic information on common respiratory infections, especially those of childhood, so that eventually preventive measures can be implemented.